The present invention relates generally to improved vehicle hub caps, and in particular, to hub caps used in truck, tractor, trailer and semi-trailer applications having special lubrication requirements.
According to the invention, an improved hub cap vent is provided to permit pressure equalization between the sealed cavity and the outside atmosphere as the mechanism is exposed to a variety of working conditions.
As is well known, the wheel hub assemblies of trucks, tractors, and semi-trailers include a spindle locating a pair of anti-friction bearings, and a wheel hub carried by the outer races of such bearings. The bearing assemblies, often referred to as the inner and outer bearing sets, are lubricated by a bath of oil or a mass of grease, depending on the application and the preference of the user.
The lubricants are prevented from leaking from the sealed region by the provision of one or more oil seals, each usually having a casing portion and an elastomeric or other style seal lip portion. Relative rotation occurs between the elastomeric seal lip and an associated, relatively rotatable part, and a static or secondary seal is created between the seal casing and a relatively fixed part of the assembly, such as a seal housing or a counterbore.
Over the years, improvements have been made in these bearing assemblies and in the seals. In some instances, there has been a widespread replacement of older style greases with oil as the lubricating medium. This change has required certain improvements in the form of seal used in the application. More recently, with the advent of improved greases, there has been an increase in the number of applications using synthetic grease instead of oil to lubricate the wheel hub assemblies.
One of the problems sought to be addressed by the present invention is the control of pressure within the sealed region formed by portions of the wheel hub assembly and the seal unit itself. In use, the seals, bearings and axles are subjected to friction as the vehicle travels over the road, gradually undergoing an increase in temperature. When heated, the air, oil and in particular, any water within the sealed region tends to expand, particularly as the liquid water turns to vapor. This exerts a positive pressure on the interior of the sealed region. If not properly vented, this pressure can increase the radial load on the seal lip and cause premature wear.
In a similar manner, when a wheel hub, once heated by reason of use, even when vented, is suddenly cooled, a partial vacuum is created in the sealed region. Under some circumstances, such as if an axle is submerged in water, the liquid water or other contaminants can be drawn into the sealed region by the action of the partial vacuum created by sudden cooling. Under these circumstances, when the vehicle is used again and the temperature rises, higher pressures can again eventuate, or as sometimes happens, the water compromises the effectiveness of the lubrication, even causing premature bearing failure.
In some instances, proper venting has overcome this problem, but in certain instances, such efforts have not been completely successful. Accordingly, there has been a need for providing proper and effective venting or breathing action of the seal cavity to ensure continuous pressure balance or equalization between the inside and the outside of the sealed region, all without leakage of the lubricant.
In the past, attempts to achieve this result have been directed to different styles of vents. One form of vent uses a standard pressure relief type valve of the spring-loaded, ball check type. Other approaches have been to use the slit elastomer or duckbill type valve to achieve a one way venting action. Still other approaches vent another portion of the mechanism, such as the inner part of the axle housing. These efforts have been successful to some extent, but some such designs may lack the ability for rapid pressure equalization upon temperature drop. In addition, a certain pressure threshold is sometimes needed to create a venting action. Regarding axle housing vents, this approach is usually applicable only to driven axles wherein the vented area communicates with the hub area, and not to all axles generally.
More recently, it has been suggested that sintered metal plugs of the type well known to be used in many breather vent or filter applications can be used for relieving pressure while also theoretically excluding the presence of contaminants from sealed regions. Materials such as sintered bronze or copper particles, preferably small sintered spheres, pressed into predetermined shapes, have been used for this purpose. In theory, it is possible to obtain interstices of a carefully controlled size that should permit the passage of gasses and water vapor but exclude liquid flow therethrough.
Proposals of this type have envisioned the use of a cylinder or plug of sintered material having a significant axial direction relative to its radial extent and, in the sense of its aspect ratio, somewhat resembling a cigarette filter. Such a sintered bronze filter is described in U.S. Pat. No. 5,192,117, for example.
While this concept appears to have several advantages, in many instances, its performance has not been as satisfactory as might have been hoped. Tests with such systems have shown that when the inner surfaces of such breather or filter becomes covered with the type of grease contained within the seal cavity, the permeability of the filter is essentially reduced to zero at the pressure levels usually encountered in most applications.
In one such filter, for example, while air flow as high as 1,000 ml per minute through a disc with an area of 0.07 sq. in. at a pressure of 5 psi (0.35 atm.) was able to be achieved while the filter was new, clean and dry, this porosity dropped essentially to zero at 5 psi when the filter was coated with a 0.050" layer of the type of grease found in the seal cavity. Where a porous plug requires a press fit within a metal opening, the plug may be distorted or rendered almost impermeable as it is wedged in place in the opening.
Accordingly, whether the filter is contacted by grease as the result of the assembly process, or merely as an incident to being used for a time, the effect has been to eliminate the venting or breathing potential of the filter and render the hub caps substantially impermeable to passage of air and water vapor at the low to moderate pressure levels encountered. Such a hub cap creates a risk of premature seal failure, with the attendant expense and inconvenience.
According to the present invention, different materials, arranged in a different way, and having a different range of porosities are provided to achieve a breathing action which is capable of being maintained even in the event of being covered with the grease used to lubricate the bearings. In one preferred form, the hub cap is made from metal and includes a breather insert in the form of a plug having a center opening, a threaded shank portion, a counterbore, and a porous breather disc received therein. The elements are made from a similar material, preferably high density polyethylene, and thus are able to be spun welded or ultrasonically bonded to each other. The material is hydrophobic and is of a controlled porosity to achieve improved performance. The vent assembly may be removed but is intended to resist accidental or unintentional removal.
In another embodiment, the vent assembly takes the form of a ceramic breather disc which is located within a transparent window portion of a hub cap having a plastic body. The disc providing the breathing action is secured within an opening in an endwall of the hub cap as the vent opening shrinks down from its slightly enlarged diameter when hot to its room temperature diameter, thus entrapping the ceramic disc in the endwall. In an alternative embodiment, the ceramic disc is placed in the counterbore of a material, such as amorphous nylon, to which ultrasonic or heat energy is thereafter applied to form a ridge or rim overlying an outer margin of the disc to secure the same in place. This method can also be used to secure the plastic disc in place within the plug forming a part of the breather insert.
In view of the failure of the prior to provide a filter unit operable at comparatively low differential pressures and permitting gas and vapor flow in both directions while preventing inward liquid flow, it is an object of the present invention to provide a hub cap having an improved vent or breather unit.
Another object of the invention is to provide a plug type breather for a hub cap wherein the breather disc is made from sintered particles, preferably spherical, of a thermoplastic material.
A further object of the invention is to provide a breather vent for a heady duty vehicle hub cap wherein the vent is made from in disc form of ceramic materials surface bonded together and providing a controlled porosity by reason of internal passages or interstices between the particles making up the major portion of the breather disc.
A still further object of the invention is to provide a method of making hub caps which include breather discs of various non-metallic materials.
Yet another object of the invention is to provide an improved hub cap made from a plastic material and having received therein a breather disc which is secured in a vent opening by a shrinkage of the plastic material between the time it is removed in a heated condition from the mold in which it is formed and the time it achieves a lower, room or operating temperature.
Another object of the invention is to provide a plug assembly for use in heavy duty hub cap wherein the breather disc is made from a porous material, preferably made from sintered particles, and carried within a plug having a center passage and an exterior shank adapted to mate with a portion of the hub cap.
A further object of the invention is to provide a breathable hub cap which will resist clogging by grease but which will also minimize or eliminate ingestion of contaminants, including water, when the unit is suddenly cooled from a high operating temperature.
The foregoing and other objects and advantages of the present invention are achieved in practice by providing a hub cap having end and sidewall portions, a vent opening, and a breather disc incorporated therein and closing off an otherwise open passage, with the breather disc being secured against removal from the end or sidewall of the hub cap and being made from a porous non-metal low cost material able to be secured in the cap wall without deformation or damage during assembly.
The manner in which the foregoing and other objects and advantages of the invention are achieved in practice will become more clearly apparent when reference is made to the following detailed description of the preferred embodiments of the invention set forth by way of example and shown in the accompanying drawings wherein like reference numerals indicate corresponding parts throughout.